In the dithering of an image, the image is divided up into a raster of image elements and the gray level of each of the image elements is determined by suitable means, see, for example, Display of Dithered Images, Proceeding of the SID, Vol. 22, No. 3, p. 185-190 (1981) which describes a method for deriving a gray-level image from its dithered version. The raster is then divided up into adjacent areas of equal size and the gray levels of the image elements in each area are compared with a dither matrix fitting on that area and consisting of a configuration of thresholds. If the gray level of an image element is greater than or equal to the corresponding threshold, that image element is given a first logic value ("1") while in the other case the image element is given the other logic value ("0"). The result of this process is a translation of the raster of gray levels into a raster of two-value pixels. These dither methods are well known in the prior art. See, for example, A Survey Of Electronic Techniques For Pictorial Image Reproduction, IEEE Transaction On Communications, Vol. COM-29, No. 12, 1898-1925 and U.S. Pat. No. 3,967,052.
Dither methods of this kind can be used to transmit image information over digital transmission lines, for example in facsimile applications, for example, German Offenlegungsschrift D.E. No. 33 44 694 A1 which describes a method for recognizing and displaying gray level image sections in a digital fax image. Dither methods can also be used to print images by means of printers which are only capable of (or suitable for) printing picture points having a single black level.
It is often desirable to perform other operations on dithered images, e.g., enlargement or reduction of the image or a part thereof. Enlargement or reduction may be desirable from the user's standpoint, while on the other hand, it may be necessary, for example, if different printers with different resolutions are used for making prints having the same format. Frequently, it is necessary to have information concerning the dither matrix with which the original image was dithered. In many cases, however, the dither matrix is unknown. Accordingly, it is an object of the present invention to provide a method for reconstructing the matrix used to generate the dither information from the available digital dither information of an image.